In systems with analog or RF complex signal paths, such as direct conversion and low-IF receiver systems, it is necessary to calibrate the system's balance of in-phase (I) and quadrature (Q) amplitude and phase signals. This is important because imbalances generate interference within the received signal by folding the negative frequencies in the complex path onto the desired signal. In a direct conversion receiver, this process causes the signal to fold onto itself, and is a well-known problem. The extent to which the interference from folding is rejected is referred to as sideband rejection.
A number of techniques have been developed to perform static I/Q calibration and to estimate the phase and amplitude imbalance. I/Q imbalance, or mismatch, is conventionally modeled as constant across the communication channel. This approximation is acceptable in narrow-band systems where the mismatch is dominated by RF contributions associated with mismatches occurring in components such as local oscillators (LO) and mixers. Such RF-based mismatches are referred to herein as static I/Q mismatches or static I/Q imbalances because they are treated as constant across the channel frequency.
However, in wideband communications systems there are mismatches that occur in the baseband circuits. Such mismatches make it difficult to achieve sideband rejection of greater than 45 dB across the band. For example, frequency-dependent mismatches in the baseband analog signal path can result in substantial degradation of sideband rejection to the 40 dB level, when 60 dB or better is required for demanding applications such as broadcast analog television applications.
FIG. 1 illustrates a simplified direct conversion receiver, as known in the prior art. I/Q mismatches introduced by LO 18's phase and amplitude differences as well as differences between I-path and Q-path mixers 22 and 12 generally result in static I/Q mismatches. Analog filter 24 disposed in the I-path, and an analog filter 14 disposed in the Q-path are adapted to reduce levels of signal spectrum that are close to the filter passband edges. Such signals often have relatively sharp transition bands, resulting in the presence of poles with high Q-factors. These poles are particularly sensitive to analog component mismatches. The resulting transfer function difference between the analog filters in the I and Q paths creates frequency-dependent mismatches. Such mismatches degrade the sideband rejection performance and continue to increase towards the filter band edge.